The results will not be released until the full poll, but the numbers of those who have voted suggests the Democrats have been successful in their campaigning.

In areas known to be heavily Democratic in the states of Georgia, Iowa, North Carolina and Ohio, people have been requesting and submitting ballots in large numbers, the Associated Press news agency reported.

Ebonee Lusk, who voted early in Fort Wayne, Indiana, said she could not wait until November 4 to cast her ballot.

"I wanted to get in, cast my vote for Barack Obama and make sure my vote counts," she said.

Obama's campaign has been targeting early voters.

"We are trying to expand the electorate and expand the process," David Plouffe, Obama's campaign manager.

In Florida, Republicans appeared to hold the edge in early voting, and McCain's campaign has said it is hoping to catch up with Obama by November 4.

Ill grandmother

Obama's campaign announced the candidate would be leaving the campaign trail for two days and cancelling some scheduled events in order see his ailing grandmother in Hawaii.

Robert Gibbs, an Obama aide, said: "Recently his grandmother has become ill and in the last few weeks her health has deteriorated to the point where her situation is very serious."

Obama intends to fly to Hawaii on Thursday and return to the campaign trail on Saturday, Gibbs said.